Results 1 to 7 of 7
  1. #1
    mtbr member
    Reputation:
    Join Date
    Jul 2004
    Posts
    544

    Crank Arm Length for 10yo?

    Hi,

    I got a 13" Bontrager race frame with a 60mm SID fork. Cool bike for a kid! But I need new cranks and must decide what length crank Arms. 165-170?

    He is 5'-0" tall, about 90lbs and a 28" floor to crotch measurement. He's a little tall for a 10yo boy, but not much outa the normasl range.

  2. #2
    mtbr member
    Reputation: kbiker3111's Avatar
    Join Date
    Apr 2005
    Posts
    322
    Depends what yer gonna do with the bike, cuz he'll probably grow.

    170 is big for 5' & 28" inseam, but if he's gonna grow, its not worth investing in two cranksets

    bravo on getting your kid a sweet bike, the 10 year old in me is jealous

  3. #3
    Mantis, Paramount, Campy
    Reputation: Shayne's Avatar
    Join Date
    Jan 2004
    Posts
    4,410

    As Short As You Can Find

    My wife is 5' 0" and rides 165's If you can find some that short go for it. Manufacturers used to make all kinds of different lenghts so you can find old oddball lengths like 165 or 167.5 for pretty cheap.

    If you want to go single ring in front you can go down to 150mm or less with BMX mini arms
    *** --- *** --- ***

  4. #4
    mtbr member
    Reputation:
    Join Date
    Jul 2004
    Posts
    544
    thanx for the replies. I am thinking that I'd like the cranks to be appropriate sized for the bike. Its a 13", so I suspect he will be on this bike for the next 2 years and then move to a 15". Looking around at the way manufacturers spec bikes, most 13" bikes have 170mm cranks. I didn't find any with 165's.

  5. #5
    jbf
    jbf is online now
    mtbr member
    Reputation: jbf's Avatar
    Join Date
    Jan 2004
    Posts
    292

    My son rides ...

    160mm Truvativ Isoflow crank arms. They came on his Ibex hardtail and they seem ideal. I have converted them to a single ring setup with a bashguard. Try this link for a rough guide for sizing cranks for kids ...

    http://www.jrbicycles.com/cranksizing.htm

  6. #6
    otb club member
    Reputation: Chris2fur's Avatar
    Join Date
    Jan 2004
    Posts
    2,226
    Quote Originally Posted by jbf
    160mm Truvativ Isoflow crank arms. They came on his Ibex hardtail and they seem ideal. I have converted them to a single ring setup with a bashguard. Try this link for a rough guide for sizing cranks for kids ...

    http://www.jrbicycles.com/cranksizing.htm
    Interesting. This note at the top is pretty relevant, though: These measurements are estimated. You can stretch the size chart to increase torque by using a slightly long crank or go to a slightly short crank for a better spin. It all depends on what you are looking for.

    I'm thinking that no matter how light you make it, the bike will be relatively heavy for a child because they are just not going to have the leg strength of an adult. The longer crank may provide them better leverage on the climbs. They can worry about perfect cadence when they get older. If you want to make changes later, you're not stuck with an oddball crankset. Just a thought...

  7. #7
    mtbr member
    Reputation:
    Join Date
    Jul 2004
    Posts
    544
    Follow up:

    My son is moving up from a 24" bike that has the 160mm Truvativ. When my son pedals that bike it looks good. When my son pedals the new bike there is a noticable difference in the range of motion.. knees are moving a large up-down pattern relative to his size.

    I looked at the sizing matrix, but that is for BMX, which generally calls for longer crank arms. Using most formula's, my son would require a 155mm crank.

    I decided to go with a set of 165's. Found a real good deal on the new XT's, which has the compact front rings which should go well with his 12-34 rear, and give him a nice range lower ratio's.

    Thanx for the input!

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •